1. Field of the Invention
The present invention relates to an electrostatic recording apparatus. More specifically, the present invention relates to an electrostatic recording apparatus wherein a photoconductive layer such as of amorphous silicon is exposed and an electrostatic latent image is formed thereon and then this electrostatic latent image is developed by a toner.
2. Description of the Prior Art
Electrostatic recording apparatus, as is well known, include a photosensitive drum and this photosensitive drum includes a photoconductive layer supported by a conductive substrate. And, charges of positive polarity are accumulated on the photoconductive layer by a charging corotron, and thereafter an electrostatic latent image is formed on the photoconductive layer by exposing the same. The electrostatic latent image is developed by using a toner.
For the photoconductive layer of such an electrostatic recording apparatus, conventionally, selenium has been frequently employed. When the photoconductive layer is of selenium, the surface potential charged by the charging corotron is 600-800 V, and to obtain such a surface potential, a current of only about 20 .mu.A is drawn into a photosensitive drum, that is, photoconductive layer from the charging corotron.
On the other hand, recently, for such a photoconductive layer of the photosensitive drum, using amorphous silicon excelling in the mechanical strength, such as wear resisting property and the like has been proposed and realized. For example, refer to the Japanese Patent Application Laid-Open No. 130038/1982 laid open on Aug. 12, 1982.
In the case where the photoconductive layer is amorphous silicon, the specific resistance thereof is smaller in comparison with that of the conventional selenium or the like, and therefore high voltage can not be kept at the photoconductive layer, and the surface potential thereof is, for instance, about 400 V. Then, in order to charge the photoconductive layer of amorphous silicon up to 400 V, usually a current of about 200-250 .mu.A is drawn into the photosensitive drum from the charging corotron.
However, when such a large current flows into the photoconductive layer, non-uniformity of charging due to non-uniform discharging of the charging corotron takes place on the photoconductive layer. Such a non-uniform charging incurs a partial breakdown in the photoconductive layer. Then, the breakdown as described above remains a permanent damage, and resultingly, no electrostatic latent image is formed at that portion, and accordingly a so-called "white blank" takes place.
Thus, in the case where the photoconductive layer whose main body is amorphous silicon is employed for the photosensitive body, a large current is required to keep the surface potential thereof at a certain value or more, while, if such a large current flows, the electric strength of the photoconductive layer is deteriorated, resulting in a short life of the photosensitive body.